Please refer to FIG. 1, which is a sectional assembled view of a conventional micropump. The conventional micropump 3 is mounted in a liquid reservoir 5 in which a cooling liquid 4 is contained. The micropump 3 includes a casing 315 a top sealing cover 32, a fan 33 and a bottom sealing cover 34. The casing 31 and the top sealing cover 32 together define a flow space 35 in which the fan 33 is accommodated. The top sealing cover 32 has a liquid inlet 351 and a liquid outlet 352. The bottom sealing cover 34 has a bottom face 341 formed with a guide groove 342. A water outlet 343 and an exhaust port 344 are formed in the guide groove 342 at an interval in communication with the flow space 35. The water outlet 343 has a diameter slightly larger than that of the exhaust port 344. One end of the guide groove 342 extends to one side of the bottom sealing cover 34 to communicate with the liquid reservoir 5. The micropump serves to expedite flowing of the cooling liquid within the liquid reservoir 5. However in such micropump 3, the casing 31, the top sealing cover 32 and the bottom sealing cover 34 are assembled to together define the closed flow space 35. Such structure has so many junctures that the tightness is relatively poor. As a result, leakage of the liquid is apt to take place. Moreover, the liquid inlet 351 of the micropump 3 is positioned on the top of the top sealing cover 32. Therefore, the cooling liquid 4 must flow into the liquid inlet 351 and then flows out of the liquid outlet 352 positioned on a circumference of the top sealing cover 32. That is, the cooling liquid 4 axially enters the micropump 3 and then radially flows out of the micropump 3. Under such circumstance, the micropump 3 has a considerable axial height as a whole. This makes it difficult to apply the micropump 3 to a site with smaller room. According to the aforesaid, the conventional micropump has the following defects:    1. The conventional micropump has higher axial height.    2. The application range of the conventional micropump is narrow.    3. The conventional micropump has poor leakproofness.